2. 计算机
  3. CSMA，although more efficient than ALOHA or slotted ALOHA，still has one glaringinefficiency．If the medium is busy，the station wil

CSMA，although more efficient than ALOHA or slotted ALOHA，still has one glaringinefficiency．If the medium is busy，the station wil

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问题 CSMA，although more efficient than ALOHA or slotted ALOHA，still has one glaringinefficiency．If the medium is busy，the station will wait for a random amount of time．Whentwo frames collide，the medium remains (71)for the duration of transmission of bothdamaged frames．The use of random delays reduces the probability of (72)．For (73)frames，compared to propagation time，the amount of wasted capacity can be considerable．This waste can be reduced if a station continues to listen to the medium while (74)．The maximum utilization depends on the length of the flame and on the (75)time；the longer theframes or the shorter the propagation time，the higher the utilization．
(71)
选项 A、convenient
B、inconvenient
C、USable
D、unusable
答案D
解析 尽管CSMA的效率远远大于ALOHA或时隙ALOHA，但它依然存在一个显著低效的情况。当信道忙的时候，站点需要等待一段随机时间。当两个帧发生冲突时，在两个被破坏帧的传输持续时间内，信道仍然无法使用。使用随机时延会降低冲突的可能性，但是如果帧的长度相对于传播时间来说很长，那么容量的浪费也是很可观的。如果站点在传输时还继续监听信道，就能减少这种浪费。最大利用率与帧长和传播时间有关，帧越长或者传播时间越短，利用率就越高。
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